Non-linear amplifiers utilizing positive feedback

Abstract

 In order to provide a constant-current driven, diode controlled logarithamic amplifier with a logarithmic response at very small input current values, the amplifier stage (12) has a feedback network (18, 24) which provides positive feedback only at low input current values.

Description

[0001] This invention relates generally to non-linear amplifiers and, more particularly, to logarithmic amplifiers employing a single amplifier stage with positive non-linear feedback.

[0002] Logarithmic amplifiers are often used in applications where there is a need to compress an input of large dynamic range into an output of small dynamic range. One means of providing the logarithmic relationship is to use a logarithmic detector, constructed from a properly biased diode matrix, and driven by a linear amplifier. Another technique is to design an amplifier that has high gain at low input levels and low gain at high input levels, thus producing a logarithmic input-output relationship.

[0003] One example of a logarithmic amplifier with a high gain at low input levels and low gain at high input levels can be found in U.S. Patent No. 3,646,456, issued to Kauffman et al., and assigned to the assignee of the present invention. Kauffman employs a plurality of non-linear amplifier stages connected in cascade. The gain of each stage is initially greater than unity and is reduced to unity upon switching of an input limiter to a high-impedance state when the input signal exceeds a predetermined amplitude.

[0004] Also known is a constant-current driven, diode-controlled logarithmic amplifier. This amplifier has an excellent logarithmic response for input currents above the value of the current from the constant current source. However, for input currents below the value of the current from the constant current source, the response tends to be linear due to the impedance of the diode increasing with decreasing input current. The rise time of the amplifier is degraded due to the capacitance and resistance associated with the input of the amplifier, and this serves to reduce the overall bandwidth of the amplifier.

[0005] In accordance with the present invention, a non-linear amplifier comprises an amplifier stage which has a non-linear element and a constant current source both coupled to its input and is provided with a feedback network coupled between its output and input for providing positive feedback from the output to the input only when the amplifier input signal is less than a predetermined value.

[0006] For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawing, the single figure of which is a schematic diagram of a logarithmic amplifier embodying the present invention.

[0007] The illustrated amplifier is a constant-current driven, diode-controlled logarithmic amplifier and comprises a non-inverting amplifier stage 12. The anode of diode 10 is connected to the input of amplifier stage 12. The cathode of diode 10 is connected to ground. Also connected to the input of amplifier stage 12 is constant current source 14, the other side of which is connected to ground.

[0008] The output of amplifier stage 12 is connected to output terminal 16. Additionally, the output of amplifier stage 12 is connected to the cathode of diode 18, the anode of which is connected to junction A. Also connected to junction A is resistor 20, the other end of which is connected to positive supply source V₁, the anode of diode 22, and one terminal of capacitor 24. The other terminal of capacitor 24 is connected to the input of amplifier stage 12. The cathode of diode 22 is connected to resistor 26, the other end of which is coupled to voltage source V₂. Diode 22 provides temperature compensation for the amplifier circuit.

[0009] In operation of the amplifier circuit, diode 10 provides a logarithmic voltage-current characteristic. The value of the current from constant-current source 14 determines the lower end bandwidth of amplifier stage 12 by setting the impedance of diode 10. In a quiescent condition, i.e., when there is no input current into the amplifier, the current through diode 10 is equal to the current from constant current source 14, and diode 18 and diode 22 are biased "on" by voltage source V₁. Additionally, the voltage potential from voltage source V2 is set to provide equal currents through diode 18 and diode 22 in the quiescent condition.

[0010] The diodes 18 and 22 are connected in series opposition and are biased to provide maximum feedback for small input signals and no feedback for large input signals. Thus, for input signals in the range where the input current I_in is less than the current supplied by constant current source 14, positive feedback is provided by way of capacitor 24 and the conduction of diodes 18 and 22. The feedback substantially linear and operates to reduce the input capacitance of amplifier stage 12 for low level input signals, thereby maintaining the bandwidth of the amplifier. When the input current I. ln increases to values in excess of the output current from constant current source 14, the impedance of diode 10 is reduced. Positive feedback is no longer needed to maintain the amplifier's bandwidth, and diode 18 becomes reverse biased and eliminates the positive feedback. In the transition zone, when the input current I_in is close to the value of the output current from constant current source 14, the positive feedback that is provided is non-linear.

[0011] The logarithmic amplifier circuit thus selectively utilizes positive feedback supplied by way of conducting diode 18 and feedback capacitor 24 to reduce the input capacitance of amplifier stage 12 for input current values less than the value of the current from constant current source 14. For input current values greater than the current from constant current source 14, diode 18 becomes reverse biased and shuts off the positive feedback. In operation the thus-controlled feedback provides a logarithmic amplifier circuit with a wide bandwidth, avoiding the instabilities normally associated with positive feedback.

[0012] The illustrated amplifier has a wide bandwidth, low noise and wide dynamic range.

Claims

1. A non-linear amplifier comprising an amplifier stage, a non-linear element coupled to the input of the amplifier stage, and a constant current source coupled to said input, characterized in that a feedback network is coupled between the output of the amplifier stage (12) and the input thereof and includes circuit elements (18, 22, 24) for providing positive feedback from said output to said input only when the amplifier input signal is less than a predetermined value.

2. An amplifier according to claim 1, characterized in that said feedback network comprises a diode (18) having its cathode connected to said output, and a capacitor (24) connected between the anode of said diode and said input, and in that the anode of the diode is connected to voltage source means (V , V ₂) whereby the diode is forward biased when the amplifier input signal is less than said predetermined value and reverse biased when the amplifier input signal is greater than said predetermined value.

3. An amplifier according to claim 2, characterized in that the voltage potential of said voltage source means (V₁, V₂) is such that said predetermined value is substantially equal to the value of the output current from said constant current source (14).

4. An amplifier according to claim 1, characterized in that said feedback network is connected to a circuit element (22) for providing temperature compensation.

5. An amplifier according to claim 4, characterized in that said feedback network comprises a diode (1₈) having its cathode connected to said output and a capacitor (24) connected between the anode of said diode and said input, and in that the temperature compensation circuit element comprises a second diode (22) connected in series opposition with the diode of the feedback network.

6. An amplifier according to claim 5, characterized in that the anode of the second diode is connected to a first voltage source (V₁) and the cathode of the second diode is connected to a second voltage source (V₂), the relative voltage potentials of the voltage sources establishing equal current flow through the two diodes when no input signal is applied to said amplifier.

Drawing